The present invention relates to separation of liquid mixtures, and in particular to a method and apparatus for separating small quantities of a liquid having a high boiling point dispersed within a large quantity of immiscible liquid having a lower boiling point.
Efficient separation of emulsified liquid mixtures is necessary in a wide range of industrial environments. For example, the lubricants employed in equipment such as gas or air compressors combine with large volumes of water during operation, necessitating disposal of the entire liquid effluent. Because the effluent is considered hazardous waste due to the relatively small amount of lubricant present therein, the ability to separate the bulk liquid into hydrocarbon and liquid fractions permits restriction of expensive disposal measures to the hazardous material itself. In addition, pre-treatment of contaminated water prior to purification frequently benefits from initial separation of hydrocarbon fractions.
Reliable techniques for separating small quantities of hydrocarbons dispersed within a large quantity of immiscible liquid are few, and the apparatus which presently perform this function provide limited efficiency. There are three types of conventional methods/apparatus for oil-water separation: (a) gravity oil-water separators; (b) dissolved air flotation; and (c) chemical treatment.
Typical gravity oil-water separators, for example, commonly employ grease-traps, baffles, skimmers and/or polishing filters to promote separation. Because they operate without phase changes, these systems require large quantities of energy for separation of free oils from the oil-water mixture, and typically cannot separate emulsified mixtures.
Dissolved air flotation is frequently used for further separation of oils from the oil-water mixture which has been previously treated by conventional oil-water separator. Chemical(s) may or may not be used.
Oil-water separation apparatus capable of operating on emulsions ordinarily utilize one or more emulsion-breaking chemicals for chemical treatment, in which either dissolved air flotation or gravity, sedimentation can be used for clarification. In general chemical treatment results in production of waste sludge that poses a solid-waste disposal problem.
The limitations associated with these oil-in-water systems derive from the respective physical properties of oil and water. The specific heat constant and heat of vaporization of water are much larger than those of typical organic compounds, including oils. Most oils boil at a higher temperature than that of water, however. Systems that rely on any form of distillation therefore require considerable energy input in order to raise the temperature of the mixture of 100.degree. C. or above, because the overall specific heat of the mixture will be close to that of water; in addition, a further quantity of energy must be applied to induce boil-off of the water.
By contrast, systems designed to remove small quantities of water from large amounts of oil require significantly less energy; this is due to the lower specific heat of the mixture (which will be closer to that of the oil), as well as to the smaller absolute amount of water that must be vaporized. Furthermore, such systems typically operate at a lower temperature than oil-in-water systems in order to minimize boil-off of organic compounds along with the water; such boil-off is rarely observed with oil-in-water system.